Continuous carburizing process



Patented Oct. 24, 1933 UNITED STATES CONTINUOUS CARBURIZING PROCESS Roysel J. Cowan, Toledo, Ohio, assignor to Surface Combustion Corporation, Toledo, Ohio, a corporation of New York No Drawing.

Application January 28, 193

Serial No. 589,539

-7 Claims.

This invention relates to a method of carburizing metal, the present application being a continuation in part of my application Ser. No.

537,873, filed May 16, 1931.

The present invention has for its object to provide a comercially practical continuous carburizing process wherein a hydrocarbon gas or vapor may be succefssfully used and withal to provide a process which among other advantages shall give (1) control of the carbon content of the case produced, (2) uniformity of results, (3) rapidity of carburization, (4) excellence of diffusion between. case and core, (5) simplicity of control of the process as a whole, and (6) cleanliness of operation.

In prior carburizing processes making use of a hydrocarbon gas, the precipitation of carbon on the work has been considered objectional and many schemes have been proposed for overcom- 20 ing the objection. In my process, however', the precipitation of carbon on the work during a certain stage of the operation is one of the essential steps of the process. However, notwithstanding the fact that I so conduct the operation as to secure deposition of carbon on the work, the work before emerging from the heat-treating or carburizing chamber is in a clean condition and in fact is almost as bright and clean. as before it was carburized. This cleanliness of the work following carburizing is something new and quite revolutionary in the art of carburizing and quite apart from other advantages of the process has distinctive merit.

Among the various steps of my process, one of the important steps is that of passing the work to be carburized and-the carburizing gas in the same direction through successive heat zones maintained at different temperatures, the first zone being the low temperature zone for reasons presently appearing.

The carburizing medium which I employ is a gaseous mixture of a hydrocarbon and a diluent containing an oxidizing agent which latter is preferably carbon dioxide although not neces- 45 sarily exclusively so.

The temperature of the low temperature zone is such as will cause decomposition of the hydrocarbon with the liberation of soot (amorphous carbon) in the presence of a catalyst, such temperature being less than carburizing temperature and ranging between 1200 and 1500 deg. F. The catalyst in the present invention is the metal to be carburized. During the passage of the work or metal through said low temperature zone, it receives a deposit of soot (amorphous carbon) resulting from the cracking or decomposition of the major portion of the hydrocarbon passing through said zone. The temperature of the high temperature zone is the usual carburizing temperature which ranges between 1550 and 1800 deg. F. Following the deposition of carbon on the work in the low temperature zone, the work with its deposit of carbon passes into the high temperature zone. In the high temperature zone, the deposited carbon on the work is completely oxidized with the formation of carbon monoxide with resultant carburization of the metal. The next step is that of carbide diffusion and such diffusion is accomplished by retaining the work in said high tem- 7o perature zone for the required period, it being understood that the high temperature zone is of such length that following oxidation of the carbon on the work, the work must still move a substantial distance .before reaching the outlet end of said zone. As soon as the work emerges from the high temperature zone, it may be directly quenched or otherwise heat-treated, the cleanliness of the work facilitating further heat= treatment. I

In order that the process may be efllciently conducted, it is essential that the precipitation of carbon in the low temperature zone occur on the work itself and not on the walls of the muflle.

To this end, the said low temperature zone or portion of the muflie is made of a non-catalytic material. So called calorized alloy, that is, iron or steel with a surface of aluminum oxide is such a non-catalytic material and may be used.

A chrome-iron alloy devoid of nickel also answers the purpose.

While the high temperature portion of the muflie may be made of the same material as the low temperature portion thereof, namely, a noncatalytic material, nevertheless it is preferred to make said high temperature portion of a material which is more-resistant to carburization, a suitable material for that purpose being chrome nickel-iron alloy.

From what has been stated, it will be readily I appreciated that from the standpoint of chemical reaction the high temperature zone may be considered as comprising two main reaction zones, namely, a carburizing zone and a diffusion zone.

The order of the zones of the heating chamber as a whole, in the order of their occurrence, are (1) a zone of carbon precipitation, (2) a carburlzing zone and (3) a diffusion zone where the carbides in the surface layer of the work diffuse inwardly towards the core of the work.

The relative proportions of hydrocarbon and diluent in the carburizing medium will be so controlled as to (l) insure the desired amount of carbon deposition on the work while it is within the low temperature zone; (2) insure the proper amount of oxidizing agent necessary to oxidize said carbon in the high temperature zone, and (3) insure an atmosphere that will not have any decarburizing efiect on the work following oxidation of said carbon and while carbide difiusion is taking place.

The hydrocarbon constituent of said carburizing medium may be any of the well known hydrocarbon gases such as manufactured city gas, nat-- ural gas, methane, ethane, propane, butane, acetylene or other saturated or unsaturated hydrocarbons having the property of decomposing with the liberation of soot when heated under proper conditions in the presence of a catalyst such as iron or steel. While I prefer to use hydrocarbon gas as such, nevertheless it is within the scope of the invention to vaporize liquid hydrocarbons and utilize the vapors in the same way that the hydrocarbon gas is used.

The diluent for the hydrocarbon is preferably dehydrated or partially dehydrated gaseous prod= nets of combustion (hereinafter also termed flue gas) whereof carbon dioxide constitutes say from 8 to 15%. The carbon dioxide constitutes the oxidizing agent for oxidizing the carbon on the work in the high temperature zone. While relatively minute quantities of water vapor and oxygen in the diluent are not harmful and may in fact be desirable, nevertheless it is preferred from the standpoint of better control of the oxidizing agent to dehydrate the flue gas. From the standpoint of uniformity of composition of the flue gas, it preferred to generate it by burning a mixture of air and gas in proportions for complete combustion of the gas.

The amount of carbon deposition on the work in the low tmperature zone can be increased or decreased by increasing or decreasing, respectively, the ratio between the diluent and the hydrocarbon. In other words, the greater the percentage of hydrocarbon in said medium the greater will be the amount of carbon deposition in a given time. The amount of carbon deposition can also be varied by varying the rate of how of said carburizing medium through the mufiie.

Until the work passes into the high temperature zone the oxidizing agent will have no appre= ciable oxidizing effect on the deposited carbon. It may, however, have some slight oxidizing effect on the work itself, but such oxidation is considered to be beneficial in the later reactions at higher temperatures.

As previously stated, the carburizing gas is caused to flow through the muiiie from the low to the high temperature zone, it being remembered that the work is also caused to move from the low to the high temperature zone.

The time required to completely oxidize said carbon on the work in the high temperature zone will, of course, depend upon the kind and amount of oxidizing agent in said diluent. Since the relative percentages of nitrogen and oxidizing agent in the diluent have no effect on the rate of carbon deposition in the low temperature zone (such rate being controlled by the relative percentages of hydrocarbon andidiluent), the rate of oxidation of said carbon on the work in the high temperature zone may be controlled by increasing the percentage of oxidizing agent in. the diluent as by increasing its carbon dioxide con== tent or by adding water vapor or oxygen or both. When said diluent is due gas the composition of the latter may be readily controlled by varying the degree of dehydration or by burning the iuel from which the hue gas is produced with an excess of air. I

As already stated the final step in the process after the deposited carbon has been oxidized with resultant carburization of the work is the difius-= ing of the carbide into the work. This is efiected by allowing the work to remain in the high tem= perature zone for an appropriate period in an at= mosphere which is non-decarburizing. In the present invention decarburization is efiectively prevented by the presence of hydrocarbon gas in the high temperature zone, it being understood that some of the hydrocarbon introduced into the mufiie does not decompose in the time available. As is well known to those skilled in the art, there is a definite amount of hydrocarbon gas required by each carbide concentration in the steel to to a neutra so-called atmosphere around the steel to prevent decarburization. Thus at a temperature of 1700 F., a steel containing l8/l00% carbon will be at equilibrium in a hydrogen atmosphere with about 25/ 100% natural gas; if the steel contains 59/ 100% carbon the equilibrium ratio is /100% natural gas while if the steel contains /l00% carbon the ratio is 1%% natural gas. Ratios higher than these will add car Icon to the steel while a lower ratio will remove carbon. The exact ratio is aflected by the temperature of the operation and the type of hydrocarbon in the atmosphere and is lowered as the temperature is raised. It is a simple matter in no the present invention to maintain the necessary hydrocarbon gas in the high temmrature zone to oifset any decarburizing efiect, it being merely necessary to increase the ratio between the hydrocarbon and the oxidizing agent in the carburizing medium admitted to the muilie.

While it is preferred to utilize flue gas as a combined diluent and oxidizing agent, neverthe less it is within the scope of the invention to prepare the diluent by g nitrogen and car- 12) bon dioxide or other oxidizing agent from separate sources of supply.

An important feature of the present invention is the manner in which any established condition dependent upon the characteristics of the car= 25 burizing mixture may be controlled and maintained. The effluent gases from the high temperature zone contain combustible constituents. By burning a constant amount of these gases in the open air in any suitable manner and utilizing 13}; a thermocouple to indicate the temperature, a constant temperature will indicate an unvarying condition. Other things being equal, a variation in temperature will indicate a change in the ratio of hydrocarbon to diluentin the mixture. If the temperature falls, it is an indication that more hydrocarbon is required to maintain the required condition in the heat treating chamber and vice versa. By proper instrumentalities operable by the temperature variations, the proportion of 14@ hydrocarbon gas and diluent in said mixture may be controlled.

While the maintenance of any particular pressure within the muflie is not essential to the suecessful operation of the process, nevertheless it M5 has been found that the rate of reaction is in= creased as the pressure is increased, and conse= quently it is within the scope of the invention to maintain such pressure within the muffle as may be desired. It of course, be derstood 15d the .purpose of heating the muille in a manner to maintain saidl temperature zones, the mume may be surrounded by a combustion chamber which may be divided by a partition to form two chambers corresponding to the high and low temperature zones, respectively, each combustion chamber being .independently fired and controlled.

As an indication of the relative lengths of the low and high temperature zones of the muiile it may be stated that in one commercial installation the low temperature portion of the muille is approximately six feet in length and the high temperature zone fourteen feet. In this installation, a case depth of 40/ 1000 inches was obtained by holding the work at a carburizing temperature of 1650 deg. F. for a period of three hours. Of said case about 10/1000 inches was hypereutectoidal. The steel used for carburizing was S. A. E. 1020, namely .20% carbon steel. To obtain said case it was found necessary to use as the carburizing medium approximately seventyfive cubic feet per hour of natural gas and seventy five cubic feet per hour of flue gas containing twelve per cent carbon dioxide. With a furnace containing a muiiie twenty feet long, exclusive of vestibules at both ends, the muflle being twenty seven inches wide by fourteen inches high, the production of carburized steel was found to be four hundred fifty pounds per hour. This rate of production is more than twice that possible with a furnace of the same length using solid carburizing medium.

What I claim as new is:

1. In the art of carburizing metal with a carburizing gas, the steps comprising passing the gas and the metal in the same direction through successive heat zones in a single chamber, the first of said zones being maintained at a lower temperature than the succeeding zone and being less active as a catalyst than steel in assisting in the thermal decomposition of a hydrocarbon gas with the liberation of soot, the said succeeding zone being maintained at carburizing temperature.

2. In the art of carburizing metal with acarburizing gas, the steps comprising passing the, gas and the metal in the same direction through successive heat zones in a single chamber, the first of said zones being maintained at a. lower temperature than the succeeding zone and being less active as a catalyst than steel in inthe thermal decomposition of a hydrocarbon gas with the liberation of soot, the said succeeding zone being maintained at carburizing temperature and the said carburizing gas comprising a hydrocarbon, an inert diluent and a gas that will oxidize carbon.

3. In the art of carburizing metal with a carburizing gas, the steps comprising passing the gas and the metal in the same direction through successive heat zones in a single chamber, the first of said zones being maintained at a lower temperature than the succeeding zone and being less active as a catalyst than steel in assisting 'in the thermal decomposition ofa hydrocarbon gas with the liberation of soot, the said succeeding zone being maintained at carburizing temperature and the said carburizing gas comprising a hydrocarbon and flue gas.

4. In the art of carburizing metal, the steps comprising first coating the metal with soot, then heating the metal to carburizing temperature and while the metal is at carburizing temperature subjecting it to the combined action of a gas comprising a gas that will oxidize the soot and a hydrocarbon gas that has previously been subjected to thermal decomposition with the liberation of carbon.

5. The method of carburizing metal which comprises first coating the metal with soot while it is coming up to carburizing temperature, such coating'being formed by thermally decomposing in the presence of the metal a hydrocarbon gas, and thereafter subjecting the metal while at carburizing temperature to the combined action of the gases remaining after the thermal decomposition of the hydrocarbon gas and a gas that will oxidize the soot.

6. The method of carburizing metal which comprises first coating the metal with soot while it is coming up to carburizing temperature, such coating being formed by thermally decomposing in the presence of the metal-a hydrocarbon gas diluted with flue gas, and thereafter subjecting the metal while at carburizing temperature to the action of the gases remaining after the thermal decomposition of the hydrocarbon gas.

7. The method 'of carburizing metal which comprises first coating the metal with soot while it is coming up to carburizing temperature, such coating being formed by thermally decomposing a hydrocarbon gas in the presence of the metal and in an environment that is less active as a catalyst than steel in assisting in such decomposition, and thereafter subjecting the metal while at carburizing temperature to the combined action of the gases remaining after the thermal decomposition of the hydrocarbon gas and a gas that will oxidize the soot.

ROYSEL J. COWAN. 

